Fluorophosphate cathodes are currently one of the most promising polyanionic sodium-ion battery cathodes and exhibit specific energies not far below oxide cathodes.To further improve fluorophosphate cathodes their capacity must be increased, which might be possible since some sodium (Na) remains unextracted in these cathodes during cycling. In this study we attempt to answer the question of what specific mechanism limits fluorophosphate cathode capacity, which could stem from either redox-limiting or site-limiting behavior. This paper reports the synthesis, electrochemical characterization, and computational examination of Na 3 GaV(PO 4 ) 2 F 3 . This test system, which was designed explicitly for uncovering the limiting factors in these structures, exhibits reversible insertion of Na + and redox activity for V 2+ through V 5+ during electrochemical cycling, indicating that fluorophosphate cathodes are not fundamentally redox-limited and must be site-limited. First-principles calculations indicate that large diffusion barriers at high sodiations impose a kinetic limit on Na + insertion in fluorophosphate cathodes, but further investigation is needed to determine capacity limits on Na + extraction. From our combined results we also propose possible routes to improve future fluorophosphate cathodes.

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This review article presents recent studies on nanostructured glass-ceramic materials with substantially improved electrical (ionic or electronic) conductivity or with an extended temperature stability range of highly conducting high-temperature crystalline phases. Such materials were synthesized by the thermal nanocrystallization of selected electrically conducting oxide glasses. Various nanostructured systems have been described, including glass-ceramics based on ion conductive glasses (silver iodate and bismuth oxide ones) and electronic conductive glasses (vanadate-phosphate and olivine-like ones). Most systems under consideration have been studied with the practical aim of using them as electrode or solid electrolyte materials for rechargeable Li-ion, Na-ion, all-solid batteries, or solid oxide fuel cells. It has been shown that the conductivity enhancement of glass-ceramics is closely correlated with their dual microstructure, consisting of nanocrystallites (5–100 nm) confined in the glassy matrix. The disordered interfacial regions in those materials form “easy conduction” paths. It has also been shown that the glassy matrices may be a suitable environment for phases, which in bulk form are stable at high temperatures, and may exist when confined in nanograins embedded in the glassy matrix even at room temperature. Many complementary experimental techniques probing the electrical conductivity, long- and short-range structure, microstructure at the nanometer scale, or thermal transitions have been used to characterize the glass-ceramic systems under consideration. Their results have helped to explain the correlations between the microstructure and the properties of these systems.

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